In recent years, metallic nanostructures have been extensively studied in the context of their optical properties.
One of the main reasons for the interest in the interaction of metallic nanostructures with light is the plasmonic property of such nanostructures in and near the visible frequency spectrum.
Plasmons are the coupled oscillations of light and free electrons at the metal-dielectric interface. These coupled oscillations result in optical fields that can be used for the propagation and localization of light beyond the diffraction limit. Among the various optical spectroscopy techniques available, Surface-Enhanced Raman Scattering (SERS) has been shown to have important applications for single-molecule sensitivity and chemical specificity.
A range of plasmonic nanostructures of different architectures have been developed in attempts to exploit their capabilities in SERS, including nano-spheres, cubes, triangles, pyramids, rods, wires and tips.
The main challenge in the use of such nanostructures is attempting to create SERS hot-spots, where the Raman scattering signal may be maximized.
Commonly, the noble metals of gold, silver or copper have been investigated for their ability to enhance the Raman scattering signal. In same instances, bimetallic materials (e.g., of gold and silver) have been studied. Various other composites utilizing a mix of silica and other noble metals have also been explored.
Fundamentally the spherical silica nanocomposite has generated keen interest in the field of biomaterials. In general, such particles may be synthesized by the addition of silicon alkoxide to a reverse water-in-oil microemulsion. The diffusion of the alkoxide into the water droplets results in the hydrolysis of the alkoxide and subsequently the formation of a oxy-hydroxy-silicate species and alcohol. A complicating factor in the use of this method is that certain bio-applications of such a nanostructure are stable over only a limited range of pH values; this may not correspond to the optimum pH for the required rapid hydrolysis and condensation reactions of the selected alkoxide.
In addition, in silica-based composites, an optimal thickness is often sought for the silica-shell layer. For example, if the silica coating within a composite SERS material is relatively thin, particularly in the presence of an alcohol solvent, the dissolution/desorption of pre-adsorbed Raman molecules at certain conditions may occur, leading to a potentially drastic decrease in the intensity of the SERS signal. Often, the Stöber process is used for the purpose of thickening the silica shell via the adoption of a tedious three-step method: (1) surface activation for silanization with coupling silane agents (2) initial silica deposition with sodium silicate in an aqueous solution and (3) extensive growth of the silica shell with a silicon alkoxide tetraethylorthosilicate) in an alcohol solution. Among the three steps mentioned, the second one is generally not well-controlled and takes a length of time (from 24 h to weeks depending on the targeted thickness) before a silica shell becomes thick enough to stabilize the particles in alcohol for the subsequent and extensive growth of silica.
There is a need to provide method for preparing a SERS particle that overcomes, or at least ameliorates, one or more of the disadvantages described above.